1. Field of the Invention
This invention relates to a photothermographic material and a method for preparing the same.
2. Prior Art
Photothermographic materials which are processed by a photothermographic process to form photographic images are disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969.
These photothermographic materials generally contain a reducible silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), a toner for controlling the tonality of silver, and a reducing agent, typically dispersed in a binder matrix. Photothermographic materials are stable at room temperature. When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, eventually forming an image.
Such photothermographic materials have been used as microphotographic and radiographic photosensitive materials.
With the recent advance of lasers and light-emitting diodes, image output devices such as laser imagers and laser image setters find widespread use. They are used for recording medical images and printing plate images. There is a strong desire to have a photosensitive material which has so high sensitivity and maximum density and is so easily dry processable that it may comply with such output devices.
The above-mentioned photothermographic materials are quite simple in that images can be formed merely by heating after exposure, and has advantages that no processing agents in liquid or powder form are required, neither peeling nor attaching step is required, and no waste is yielded. Because of these advantages, the photothermographic materials are regarded potentially suitable for use in laser output devices.
Prior art photothermographic materials are generally prepared by dissolving a binder in an organic solvent, dispersing an organic silver salt and silver halide in the binder, adding a solution of a reducing agent and toner in a similar organic solvent to the dispersion, and applying the resultant coating solution to a film support, followed by drying. This process has several problems of (1) environmental pollution that the organic solvent is evaporated in the coating and drying steps to diffuse into the air, (2) low productivity that the coating rate is low and concurrent coating of multiple layers is difficult and (3) hazard including flammability and explosion.
To solve these problems, we attempted to design a photothermographic material as an aqueous system using a water-soluble binder, but failed to provide satisfactory photographic performance.
For example, JP-A 52626/1974 and 116144/1978 disclose the use of gelatin as a binder. JP-A 151138/19775 discloses the use of polyvinyl alcohol as a binder. JP-A 61747/1985 discloses the combined use of gelatin and polyvinyl alcohol. JP-A 28737/1983 discloses a photosensitive layer containing water-soluble polyvinyl acetal as a binder. The use of these binders leads to environmental and economical benefits because a photosensitive layer can be formed using a coating solution in a water solvent.
Photosensitive materials using gelatin, polyvinyl alcohol, polyacetal and other water-soluble polymers as the binder, however, have the drawback that fog is increased when they are stored in a humid atmosphere. It is thus desired to have a technique capable of forming a photosensitive layer from an aqueous system which is advantageous from environmental and economical aspects and suppressing fog upon storage in a humid atmosphere.